The invention relates to current interrupters of the vacuum-type for use in controlling fault currents associated with transmission lines in power distribution systems.
Increased electric power demand has led utility systems to use higher voltages in the transmission of power. Fault currents, due to ground shorts for example, can rapidly increase on high voltage lines. Therefore, as transmission voltages rise there is a continuous need in the electric power industry for improved current limiting devices capable of rapidly controlling high fault currents.
One type of current limiting circuit employs a vacuum-type current interrupter in parallel with a current-suppressive load. Such an interrupter employs a pair of relatively movable electrodes within an evacuated envelope. The movable electrodes can be placed in electrical contact to provide a free path for current flow. When excessive current is detected the electrodes are rapidly separated. Arcing then occurs across the interelectrode gap. The arcing causes a cloud of metallic vapor to arise within the evacuated envelope, the vapor eventually being deposited on the interior walls of the envelope.
In a standard current interrupter the evacuated envelope is cylindrical in shape with the electrodes being supported from the ends. The cylindrical portion is formed of an insulating material such as ceramic or glass. The end caps which support the electodes are metal. In order to successfully interrupt the flow of current, no conductive path can exist between the end caps along the insulated cylindrical portion. However, each time an interrupter is used, metallic arcing vapor deposits accumulate on the interior surface of the insulating wall. This metallic buildup can eventually cause current paths between the end caps which either short out the interrupter or severely limit the transient voltages it can withstand.
One means of preventing metallic vapor deposits on the interior walls of the insulation portion is to suspend a metal arcing shield within the envelope. The shield surrounds the electrodes so that the arcing vapors are deposited on the shield, rather than on the insulating walls. The metal shield prolongs the life of the interrupter but presents problems when used in interrupters employing magnetic arc suppression.
It has been found that the presence of a metal arc shield around the electrodes within the vacuum envelope tends to degrade the performance of a magnetic arc suppression interrupter. It is known, for example, that in interrupters without such a shield the maximum interruptable current is a function of the interelectrode magnetic flux density. Increasing the strength of the transverse magnetic field leads to improved performance. With a metal shield suspended in the envelope, however, it has been found that performance increases with the strength of the magnetic field only to a point, after which further increases in magnetic field strength actually tends to decrease performance. This is thought to be due to the presence of eddy currents generated within the metal arc shield. Another problem associated with the metal arc shield is that it provides additional current paths transverse to the lines of magnetic force. It is only when arcing proceeds transverse to the lines of magnetic force that magnetic arc suppression is effective. If arcing instead occurs along the lines of magnetic force, from the electrode to the shield and back to the other electrode, the magnetic field is ineffective to extinguish the arc. It is therefore most desirable when using magnetic arc suppression to dispense with the metal shield.